In intaglio printing presses, it is commonly known to use a wiping cylinder contacting the plate cylinder carrying the intaglio printing plate or plates as a wiping device for wiping and cleaning the surface of the intaglio printing plate or plates. The purpose of such a wiping cylinder is to simultaneously press the ink deposited onto the printing plates into the engravings and clean the excess ink from the plenum of the printing plates, i.e. the unengraved area of the printing plates outside the engravings.
In order to achieve good printing quality, the wiping cylinder is commonly designed in such a way that its outer surface contacting the printing plates is both physically and chemically resistant, i.e. is adapted to sustain the high contact pressure and friction with the printing plates and can withstand the physical and chemical contact with the ink components and pigments, as well as with the cleaning solutions which are used to clean the surface of the wiping cylinder.
It has already been proposed to provide such a wiping cylinder with an outer layer of resilient synthetic composition, namely a heat-hardenable plastic composition such as PVC. U.S. Pat. Nos. 3,785,286, 3,900,595 and 4,054,685 for instance disclose methods for making such wiping cylinders as well as apparatuses for implementing the said methods. These publications are incorporated by reference in the present application, especially in respect to the material used for forming such cylinders and to the machines and methods used for building such wiping cylinders. Referring for instance to the coating apparatus described in U.S. Pat. No. 4,054,685, means are provided for mounting a cylinder to be coated for horizontal rotation about its axis of rotation. Coating is performed by rotating the cylinder past a coating unit consisting of a straight-edged scraper blade mechanism disposed at one side of the cylinder and which extends parallel to the cylinder axis, this blade mechanism being adapted to be moved towards and away from the cylinder. The blade mechanism consists of two blades mechanically coupled to each other, namely a lower blade and an upper blade which are jointly designed to ensure a proper supply of heat-hardenable plastic material to the surface of the cylinder to be coated and allow adjustment of the thickness of the material to be deposited. The blade mechanism is adapted to be moved towards and away from the cylinder while maintaining the straight edge of the lower blade (i.e. the edge which extends along the length of the cylinder) parallel to the axis of rotation of the cylinder. The plastic material is supplied to the blade mechanism on top of the upper blade which is disposed, during coating of the cylinder, in an inclined relationship with respect to the cylinder so as to form a reservoir between the upper side of the upper blade and the periphery of the cylinder to be coated. Means are provided for restraining flow of the plastic material sideways from the reservoir. The blade mechanism can be translated towards and away from the cylinder in order to maintain a desired uniform spacing (a couple of millimeters or less) between the straight edge of the lower blade and the periphery of the cylinder along the full length of the cylinder. The cylinder is rotated in a direction to cause its periphery to move downwardly past the blade mechanism to thereby apply to the periphery of the cylinder a thin uniform layer of plastic composition having a thickness determined by the spacing between the straight edge of the lower blade and the periphery of the cylinder. This layer of plastic material is heat-cured by applying radiant heat to the cylinder throughout its length as the cylinder is rotated so as to cause hardening of the deposited layer of plastic material and produce a hardened layer of the desired hardness. Several layers with different hardnesses and thicknesses are preferably formed in this way onto the cylinder surface.
According to the solutions described in U.S. Pat. No. 4,054,685, radiant heat is applied to the cylinder by heating elements (such as heating lamps or resistor elements) which extends along the length of the cylinder and around at least part of the periphery of the cylinder. The position of these heating elements can be adjusted manually with respect to the position of the cylinder in order to obtain a substantially uniform heat distribution over the whole length of the cylinder. Before the coating process, a pyrometer is used to control the temperature distribution along the cylinder, the pyrometer being displaced manually in front of the cylinder. Once the initial adjustment of the heating elements has been performed, the pyrometer remains stationary in a mid-position and functions as a sensor for the automatic heating control whereby temperature and time are controlled according to a predetermined program.
One disadvantage of the above solution resides in the fact that each heating element extends along the whole length of the cylinder and in that heating control cannot be performed in a very precise manner along the length of the cylinder, especially at the two ends of the cylinder where temperature can fluctuate by a substantial amount due to edge effects caused by the rotation of the cylinder and the flow of air around the cylinder. Further, heating control is performed based on a local measurement of the surface temperature of the cylinder, i.e. at a mid-position, which does not precisely reflect the temperature profile along the whole length of the cylinder.
U.S. Pat. No. 5,180,612 discloses another coating apparatus which is fitted with a plurality of discrete heating elements (such as ceramic tiles) arranged in a matrix of five or six rows of eight elements, each row extending along the length of the cylinder. Each tile is curved to present a concave surface which is directed towards and somewhat follows the curvature of the cylinder. The tiles are mounted at their rear end onto a stainless steel reflector mounted inside a hood part that can be pivoted onto or away from the cylinder mounting location.
Electrical power to each tile can be independently switched by a matrix panel of push buttons with internal illumination capability such that those tiles which are switched on at any instant are indicated by the illumination of the corresponding push button. The heating profile is thus displayed by the illumination states of the push-buttons on the matrix panel. Further, the amount of electrical power fed to the various tiles is controlled in dependence upon the outputs of three non-contact IR temperature sensors which monitor the temperature of the surface of the cylinder. More precisely, left-hand side and right-hand side outer sensors monitor all three, two or the outermost one of the outer circumferential columns of tiles at the left-hand and at the right-hand ends of the matrix, respectively. These columns of the matrix are thus independently controlled or isolated by the outer located sensors. The remaining one of the eight columns of tiles, in the middle of the matrix, that is the fourth and fifth columns, or the third to sixth columns, or the second to seventh columns, are capable of being electrically controlled by a centrally positioned sensor.
A disadvantage of this solution resides in the fact that heating control cannot again be performed in a very precise manner along the length of the cylinder. While the provision of three separate sensors helps in achieving a more uniform control of the heating profile, the proposed control scheme is still insufficient. Indeed, at least one sensor (either the central sensor or each one of the outer sensors) controls a plurality of columns of heating elements, a common temperature measurement being apparently used to adjust the heating power of all the columns of heating elements associated to that sensor. This again is not a satisfying solution because heating control is based on a local measurement of the surface temperature of the cylinder which does not precisely reflect the temperature profile along the portion of the length of the cylinder that is subjected to the heating produced by the corresponding group of columns of heating elements.
Another disadvantage of this solution resides in the fact that the proposed configuration imposes constraints as to the location of the cylinder with respect to the heating elements and the sensors. Indeed, as three sensors are used to monitor the surface temperature of the cylinder at the left-hand side, the middle part and the right-hand side, respectively, the cylinder to be coated must be located so that its mid-point faces more or less precisely the centrally-located sensor and so that the outer sensors are still capable of reading the surface temperature of the outer zones of the cylinder. In addition, depending on the length of the cylinder to be processed, one has to ensure that the outer columns of heating tiles which emit IR radiations do not interfere with the outer sensors. This implies either the complete switching-off of outer columns of heating elements and/or locating the outer sensors in such a manner that they do not directly face the heating tiles that are not or partly hidden behind the cylinder.